Nyholm, Sten E.

Abstract [en]

In order to gain experience in explosive pulsed power and to provide experimental data for modeling, a small high-explosive-driven helical magnetic flux-compression generator (FCG) was designed at the Swedish Defence Research Agency (FOI). The generator, of which three have been built, has an overall length of 300 mm and a diameter of 70 mm. It could serve as the energy source in a pulse-forming network to generate high power pulses for various loads. This paper presents the design of, and tests with, this helical FCG. The generator had an initial inductance of 23 mu H and was operated into a load of 0.2 mu H. The generator is charged with 0.27 kg of high explosives (PBXN-5). Various types of diagnostics were used to monitor the operation of the generator, including current probes, optical fibers, and piezo gauges. With seed currents of 5.7 and 11.2 kA, final currents of 269 and 436 kA were obtained, corresponding to current amplification factors of 47 and 39. The peak of the current was reached about 30 mu s after the time of crowbar. The two generators showed only small losses in terms of 2 pi-clocking. Using signals from optical fibers, the deflection angle of the armature could be determined to be 10 degrees in good agreement with hydrodynamic simulations of the detonation process and the detonation velocity to be 8.7 km/s in agreement with tabulated value.

Larsson, Anders

Novac, Bucur

Nyholm, Sten E.

Abstract [en]

In order to gain experience in explosive pulsed power and to provide experimental data as the basis for computer modeling, a small high-explosive-driven helical magnetic flux-compression generator (FCG) was designed at the Swedish Defence Research Agency. The generator, of which three have been built, has an overall length of 300 mm and a diameter of 70 mm. It could serve as the energy source in a pulse-forming network to generate high-power pulses for various loads. This paper presents a simulation model of this helical FCG. The model, which was implemented in Matlab-Simulink, uses analytical expressions for the generator inductance. The model of resistive losses takes into account the heating of the conductors and the diffusion of the magnetic field into the conductors. The simulation results are compared with experimental data from two experiments with identical generators but with different seed currents, influencing the resistive losses. The model is used to analyze the performance of the generator.

Larsson, Anders

Nyholm, Sten E.

Abstract [en]

A research activity involving the detailed consideration of novel high-voltage transformers for pulsed-power applications has recently begun at Loughborough University. Although the main goal is the demonstration of a compact and lightweight unit employing magnetic insulation under vacuum conditions, the initial stage of the work is directed toward the development of a conventional air-cored transformer as a main component in a compact power supply for high power microwave sources. In cooperation with the Swedish Defence Research Agency (FOI), the power supply has been tested with a microwave source of the vircator type. The power generator for the system uses a 70-kJ/25-kV capacitor bank and an exploding wire array to generate a 150-kV voltage pulse in the primary circuit of a transformer. A pressurized SF6 spark gap in the secondary circuit sharpens the high-voltage output so that pulses approaching 500 kV and with a rise time well below 100 ns are generated on a 20-Omega high-power resistors With the peak power in excess of 10 GW. Experimental results are presented from final testing of the system, where a reflex triode vircator replaces the 20-Omega resistor. Measurements of the microwave emission using free-field sensors are presented, and comments are made with the microwave emission from the same vircator when powered by a Marx generator at FOI.

Westerling, Lars

Larsson, Anders

Hurtig, Tomas

Abstract [en]

In this paper we present a study of the interaction between all electric current pulse and a solid copper jet. Experiments were performed using a dedicated pulsed power supply delivering a current pulse of such amplitude, rise little, and duration that the jet is efficiently affected. The copper jet was created by using a shaped charge warhead. All electrode configuration consisting of two aluminum plates with a separation distance of 150 mm was used. The discharge current pulse and the voltages at the capacitors and at the electrodes were measured to obtain data oil energy deposition in and the resistance of the jet and electrode contact region. X-ray diagnostics were used to radiograph the jet, and by analyzing the radiograph, the degree of disruption of the electrified jet could be obtained. It was found that a current pulse with an amplitude of 200-250 kA and a rise time of 16 mu s could strongly enhance the natural fragmentation of the jet. In this case, the initial electric energy was 100 kJ and about 90% of the electric energy was deposited in the jet and electrodes. At the exit of the electrode region, the jet fragments formed rings with a radial velocity of up to 200 m/s, depending oil the initial electric energy in the pulsed power supply. [DOI: 10.1115/1.3172251]

Larsson, Anders

Lundberg, Patrik

Skoglund, Melker

Westerling, Lars

Abstract [en]

The interaction between a solid copper jet and an electric current pulse is studied. Copper jets that were created by a shaped-charge device were passed through an electrode configuration consisting of two aluminum plates with a separation distance of 150 mm. The electrodes were connected to a pulsed-power supply delivering a current pulse with amplitudes up to 250 kA. The current and voltages were measured, providing data on energy deposition in the jet and electrode contact region, and flash X-ray diagnostics were used to depict the jet during and after electrification. The shape of, and the velocity distributions along, the jet has been used to estimate the correlation between the jet mass flow through the electrodes and the electrical energy deposition. On average, 2.8 kJ/g was deposited in the jet and electrode region, which is sufficient to bring the jet up to the boiling point. A model based on the assumption of a homogenous current flow through the jet between the electrodes underestimates the energy deposition and the jet resistance by a factor 5 compared with the experiments, indicating a more complex current flow through the jet. The experimental results indicate the following mechanism for the enhancement of jet breakup. When electrified, the natural-formed necks in the jet are subjected to a higher current density compared with other parts of the jet. The higher current density results in a stronger heating and a stronger magnetic pinch force. Eventually, the jet material in the neck is evaporated and explodes electrically, resulting in a radial ejection of vaporized jet material.

Lundberg, Patrik

Westerling, Lars

Larsson, Anders

Hurtig, Tomas

Abstract [en]

Interaction between a solid copper jet and an electric current pulse is a complex process that has been experimentally studied by letting a jet created by a shaped charge device pass through an electrode configuration consisting of two aluminum plates with a separation distance of 150 mm. When the jet bridged the electrodes, which are connected to a charged pulsed power supply, current pulses with amplitude up to 250 kA were passed through the jet. By using flash X-ray diagnostics, the disruption of the electrified jets could be studied. In this paper, the disruption of the electrified jets is discussed and compared with disruption phenomena observed in electrically exploded metal rods in a static setup. Necks are naturally formed along a stretching jet, and in the experiments with current interaction these necks explode electrically. In the static experiments, the metal rods have small notches distributed along the rod to resemble the necks of the jet. When two neighboring necks or notches explode, the shock of the explosion compresses the intermediate jet or rod segment axially and the material is forced out radially. The disruption phenomena in the jet and rod experiments are similar with rapid expansion of the metal at explosion and at comparable velocities.

Larsson, Anders

Lundberg, Patrik

Skoglund, Melker

Westerling, Lars

Abstract [en]

This paper presents experimental studies of current interaction with static copper rods. The results of the static experiments are used to discuss the effects responsible for the disruption of shaped-charge jets in electric armour. These effects include ohmic heating of, and current diffusion into, the rod as well as radial magnetic forces.